linux-kernel - Re: [PATCH] mm: readahead: improve mmap_miss heuristic for concurrent faults

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Message-ID: <nbhyazac6tig3u753upst4brgmlo6qlwdntxdgwtfslpmmvp6h@gakfswzn3p75>
Date: Mon, 25 Aug 2025 10:16:40 +0200
From: Jan Kara <jack@...e.cz>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] mm: readahead: improve mmap_miss heuristic for
 concurrent faults

On Fri 15-08-25 11:32:24, Roman Gushchin wrote:
> If two or more threads of an application faulting on the same folio,
> the mmap_miss counter can be decreased multiple times. It breaks the
> mmap_miss heuristic and keeps the readahead enabled even under extreme
> levels of memory pressure.
> 
> It happens often if file folios backing a multi-threaded application
> are getting evicted and re-faulted.
> 
> Fix it by skipping decreasing mmap_miss if the folio is locked.
> 
> This change was evaluated on several hundred thousands hosts in Google's
> production over a couple of weeks. The number of containers being
> stuck in a vicious reclaim cycle for a long time was reduced several
> fold (~10-20x), as well as the overall fleet-wide cpu time spent in
> direct memory reclaim was meaningfully reduced. No regressions were
> observed.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: Jan Kara <jack@...e.cz>
> Cc: linux-mm@...ck.org

Looks good! Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  mm/filemap.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index c21e98657e0b..983ba1019674 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3324,9 +3324,17 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
>  	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
>  		return fpin;
>  
> -	mmap_miss = READ_ONCE(ra->mmap_miss);
> -	if (mmap_miss)
> -		WRITE_ONCE(ra->mmap_miss, --mmap_miss);
> +	/*
> +	 * If the folio is locked, we're likely racing against another fault.
> +	 * Don't touch the mmap_miss counter to avoid decreasing it multiple
> +	 * times for a single folio and break the balance with mmap_miss
> +	 * increase in do_sync_mmap_readahead().
> +	 */
> +	if (likely(!folio_test_locked(folio))) {
> +		mmap_miss = READ_ONCE(ra->mmap_miss);
> +		if (mmap_miss)
> +			WRITE_ONCE(ra->mmap_miss, --mmap_miss);
> +	}
>  
>  	if (folio_test_readahead(folio)) {
>  		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
> -- 
> 2.50.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR